1. Field of the Invention
This invention relates to the diagnosing of faults in an image forming apparatus.
2. Description of the Related Art
When a component part in an image forming apparatus using electrophotography develops a fault or undergoes a decline in durability, the image formed loses density and develops fogging and density unevenness. For example, with regard to uneven density, it is required that a photosensitive drum be charged or electrified uniformly by corona discharge in a charging step and de-electrified uniformly in an exposure step in order to obtain a uniform reproduction image. However, the corona charging device is electrified unevenly owing to contamination by stray developer in the apparatus, and a non-uniformity in the amount of exposing light is produced by deterioration of the exposure lamp through aging. When these phenomena occur, the potential distribution on the photosensitive drum becomes non-uniform and is a cause of uneven density. Further, it goes without saying that the charging step, the deterioration of the exposure lamp and malfunctions are a cause of lighter density and fogging.
To discriminate the cause of lighter density, fogging and uneven density, a serviceman performing maintenance makes a judgment upon observing the image formed on recording paper. Further, self-diagnosis is carried out by using a density monitor and potential sensor to monitor the density of the toner image on the photosensitive drum as well as the potential of the latent image. A method of self-diagnosis involves providing a threshold value for the status quantity of each process element within the apparatus and determining the location of a fault using this threshold value as a reference.
In the conventional technique for diagnosing the location of a fault, the status quantity for specifying the cause of the fault and the fault location are in 1:1 correspondence, and whether or not a fault is present is judged by measuring the status quantity that is the cause of the fault. In order to make this judgment, the method used involves providing a threshold value for each status quantity and judging that a fault has occurred only if the prescribed threshold value is exceeded. However, in fault diagnosis, such as in control management of output-image density, for a case in which malfunction and/or aging of each process element takes place comparatively gradually, there are instances in which the diagnostic results are erroneous or in which extreme results are outputted when the conventional diagnostic technique is used. Furthermore, owing to the 1:1 correspondence between the status quantity for specifying the cause of the fault and the fault location in accordance with the conventional diagnostic technique, it is difficult to diagnose a plurality of fault locations while observing interrelationships using a plurality of status quantities.
By way of example, the cause of uneven density involves a variety of factors and these factors are interrelated in complex ways. Accordingly, in judging cause, it is required that the serviceman have a high level of knowledge and experience. There is also the risk of erroneous judgments being made. In addition, in order to perform self-diagnosis associated with each of the sensors, it is required that a complicated decision program be written based upon a large quantity of experimental data. Since there are many causes of uneven density, as mentioned above, it is necessary to experimentally obtain the relationship between a change in the degree of deterioration of each part that is a cause candidate and a change in uneven density in order to create the aforementioned program. More specifically, a voluminous experimental-data table is required before the program is written, and compiling the table necessitates a large amount of time and labor. In actual practice, therefore, often only the especially important candidates among the large number thereof are taken into consideration.
In order to satisfy the need for an improvement in the reliability of an image forming apparatus, it is required that judgment be automated instead of relying upon the aforementioned visual judgment of the image. In addition, it is necessary that information regarding a large number of status quantities be taken into account and that the method of judgment be one based upon a simple decision program.
Pre-transfer, transfer and corona charging for separation in an image forming apparatus involves processes for externally applying electric charge to the toner image on a photosensitive drum, transferring the toner image to transfer paper and peeling off the transfer paper from the photosensitive drum. In a high-speed machine, adjustment values in each of the processes are important and delicate quantities that influence the performance of the apparatus. Accordingly, in conventional practice, such factors as the characteristics of the toner on the photosensitive drum (namely the amount of electric charge on the toner), the quantity of toner (which is dependent upon the state of the original), the type of transfer paper, the water content of the transfer paper, the conveyance speed of the main body of the apparatus, the contamination of each of the charging devices and the differences between machines are taken into account, and the set values of the charging devices are obtained by the repetition of complicated experiments.
In general, however, deviations in the above-mentioned factors cannot be covered by a single set value, and it is necessary to change over the output level by a service man, to make adjustments, to perform maintenance of the charging devices and the like and to check the transfer paper.
However, in the case of malfunctions and problems related to transfer and separation, a variety of causes are conceivable and it is required that the serviceman possesses a high-degree of knowledge and experience. In addition, apparatus downtime is prolonged and locations different from those that are faulty may be adjusted inadvertently.